1. Field of the Invention
The present invention relates to a television receiver and, more particularly, to a television receiver of a synchronous carrier reproducing system.
2. Description of the Prior Art
Superheterodyne television tuners have been used for conventional television receivers or as VTR tuners.
FIG. 1 is a block diagram showing a main part of a conventional superheterodyne television receiver. Referring to FIG. 1, reference numeral 102 denotes an RF amplifier; 104, a mixer; 106, an local oscillator; 108, an IF amplifier; 110, a detector; and 112, an output terminal.
An operation of the television receiver having the above-described arrangement will be described below. If the frequency of a desired signal to be applied to the RF amplifier 102 is represented by f.sub.RF, and the local oscillator 106 frequency is represented by f.sub.LO, an IF signal having an intermediate frequency f.sub.IF =f.sub.LO -f.sub.RF is obtained from the mixer 104. In this case, an image signal having a frequency f.sub.IM =f.sub.LO +f.sub.IF is rejected by a tuned circuit in the RF amplifier 102. The IF signal is amplified by the IF amplifier 108 and is demodulated by the detector 110. As a result, a baseband video signal is obtained from the output terminal 112.
With such an arrangement, the following advantages can be provided: (1) good frequency selectivity can be obtained; (2) a high gain is ensured prior to the detector without oscillation; and hence (3) the detector can be operated at a relatively high level to obtain good linearity.
In the above-described arrangement, an electronic tuner using a varactor diode is widely used as a tuning element of the RF amplifier 102. In this arrangement, the loss of the tuned circuit in RF amplifier is increased due to a series resistance component of the varactor diode. If required rejection at image frequency is to be obtained, the noise figure is greatly degraded. In order to improve the noise figure, the image rejection must be reduced. This inevitably causes insufficient removal of an image frequency.
In order to prove the above-described problem, a synchronous receiver of a synchronous carrier reproducing system having a phase locked loop known as a Costas loop is applied to a television receiver.
FIG. 2 is a block diagram showing a main part of a conventional television synchronous receiver of a synchronous carrier reproducing system. This receiver comprises: an RF amplifier 202 for amplifying a modulated video carrier wave; a first synchronous detector 204 for detecting an in-phase component of the output from the amplifier 202; a second synchronous detector 206 for synchronously detecting an orthogonal component of the output; low-pass filters 208 and 210 for allowing only baseband signal components of the outputs from the two synchronous detectors to pass therethrough; video signal amplifiers 212 and 214 for amplifying the baseband video signal components; a phase detector 224 for comparing the phases of the outputs from the two signal amplifiers; a low-pass filter 222 for allowing only a low-frequency component of the output from the phase detector 224 to pass therethrough; a voltage adder 220 for adding an output from a channel selection voltage generator 226 for generating a channel selection voltage to the output voltage from the low-pass filter 222; a voltage controlled oscillator 216 to be controlled by the output from the voltage adder 220; a 90.degree. phase shifter 218 for phase-shifting the signal from the voltage controlled oscillator 216 by 90.degree.; and a video signal filter 228. The detected baseband video signal is obtained from an output terminal 230.
In this case, an I signal block 250 is a circuit block, constituted by the synchronous detector 204, the low-pass filter 208, and the video signal amplifier 212, for detecting an in-phase component of a modulated video carrier wave, and a Q signal block 252 is a circuit block, constituted by the synchronous detector 206, the low-pass filter 210, the video signal amplifier 214, and the 90.degree. phase shifter 218, for detecting an orthogonal component of the modulated video carrier wave.
An operation of each signal block will be described below.
A modulated carrier wave of received channel input to the RF amplifier 202 is modulated into a vestigial sideband wave, as shown in FIG. 3. This vestigial sideband wave is divided into three regions, as shown in FIG. 3, and the process of detecting the modulated carrier wave of received channel will be described below, considering the I and Q signal blocks separately.
Referring to FIG. 3, reference numeral 270 denotes an AM (Amplitude Modulation) region; 272, a DSB (Double Sideband) region; and 274, an SSB (Single Sideband) region. Reference symbol Vr denotes a picture carrier; and Ar, a sound carrier.
The I signal block will be described first.
A modulated carrier wave V.sub.rAM (t) in the AM region 270 in the received channel is represented as EQU V.sub.rAM (t)=A(1+kcospt) cos (.omega..sub.C t-.alpha.) (1)
where A is the amplitude, k is the modulation factor, cospt is a baseband video signal (0.ltoreq.p/2.pi..ltoreq.750 kHz), .omega..sub.C is the angular frequency of a picture carrier, and .alpha. is the initial phase of the picture carrier.
An output signal (synchronous carrier) V.sub.LOI (t) of the voltage controlled oscillator 216 is represented as EQU V.sub.LOI (t)=B cos (.omega..sub.L t-.beta.) (2)
where B is the amplitude, .omega..sub.L is the angular frequency of an oscillation output signal, and .beta. is the initial phase of the oscillation output signal.
When the output signal from the RF amplifier 202 and the output signal from the voltage controlled oscillator 216 are applied to the synchronous detector 204, an output VBAMI(t) from the detector 204 is represented as follows according to equations (1) and (2): ##EQU1## The term .omega..sub.C +.omega..sub.L is eliminated by the next low-pass filter 208, and the output from the filter 208 is given by ##EQU2##
Since control is performed to establish .omega..sub.C =.omega..sub.L and .alpha.=.beta. by the Costas loop, i.e., the phase locked loop, constituted by the channel selection voltage generator 226, the I signal block 250, the Q signal block 252, the phase detector 224, the low-pass filter 222, the voltage adder 220, and the voltage controlled oscillator 216, equation (4) can be rewritten as ##EQU3## In this manner, the baseband signal is demodulated.
A modulated carrier wave in the DSB region 272 in the received channel will be described below. Since the television signal is modulated into the vestigial sideband wave, part of the lower sideband wave of the modulated carrier wave in the DSB region 272 is attenuated. If this attenuation coefficient .eta. (0.ltoreq..eta..ltoreq.1) is introduced, a modulated carrier wave V.sub.rDSB (t) in the DSB region 272 can be represented as ##EQU4## where 750 kHz&lt;p/2.pi..ltoreq.1.25 MHz
An output V.sub.BDSBI (t) from the synchronous detector 204, therefore, is given by ##EQU5## Since the terms .omega..sub.C +.omega..sub.L -p and .omega..sub.C +.omega..sub.L +p are eliminated by the next low-pass filter 208, ##EQU6## Similar to the signal in the AM region, since control is performed by the phase locked loop to establish .omega..sub.C =.omega..sub.L and .alpha.=.beta. for the signal in the DSB region, equation (8) is rewritten as ##EQU7## In this manner, a demodulated signal is obtained.
A modulated carrier wave in the SSB region 274 of the received channel will be described below. A modulated carrier wave V.sub.rSSB (t) in the SSB region 274 is represented as ##EQU8## for 1.25 MHz&lt;p/2.pi.&lt;4.5 MHz.
An output V.sub.BSSBI (t) from the synchronous detector 204 is given by ##EQU9##
Since the term .omega..sub.C +.omega..sub.L +p is eliminated by the next low-pass filter 208, an output from the filter 208 is given by ##EQU10##
Similar to the signal in the AM and DSB regions, since control is performed by the phase locked loop to establish .omega..sub.C =.omega..sub.L and .alpha.=.beta. for the signal in the DSB region, equation (12) is rewritten as ##EQU11## In this manner, a demodulated signal is obtained.
As a result, a baseband video signal cospt is demodulated in the I signal block according to equations (5), (9), and (13).
A detection process in the Q signal block will be described below in the same manner as in the I signal block.
Since an output signal from the voltage controlled oscillator 216 is phase-shifted by the 90.degree. phase shifter 218, an output V.sub.LOQ (t) from it can be represented as ##EQU12## Therefore, in the AM region 270 of the received channel, an output V.sub.BAMQ (t) from the synchronous detector 206 is represented as follows according to equations (1) and (14): ##EQU13## The term .omega..sub.C +.omega..sub.L is eliminated by the next low-pass filter 210, and an output from the filter 210 is given by ##EQU14## Since control is performed by the phase locked loop to establish .omega..sub.C =.omega..sub.L and .alpha.=.beta., equation (16) is rewritten as EQU V.sub.BAMQ (t)=0 (17)
A signal in the DSB region 272 is calculated in the same manner as in the I signal block. As a result, an output V.sub.BDSBQ (t) from the synchronous detector 206 is represented as follows according to equations (6) and (14): ##EQU15## Since the terms .omega..sub.C +.omega..sub.L -P and .omega..sub.C +.omega..sub.L +p are eliminated by the low-pass filter 210, equation (18) is rewritten as ##EQU16## Similarly, in this case, since control is performed by the phase locked loop to establish .omega..sub.c =.omega..sub.L and .alpha.=.beta., equation (19) is rewritten as ##EQU17##
A signal in the SSB region 274 is calculated by the same process as described above. As a result, an output V.sub.BSSBQ (t) from the synchronous detector 206 is given by follows according to equations (10) and (14): ##EQU18## Since the term .omega..sub.C +.omega..sub.L +p is eliminated by the next low-pass filter 210, equation (21) is rewritten as ##EQU19## Similarly, since control is performed by the phase locked loop to establish .omega..sub.C =.omega..sub.L and .alpha.=.beta., equation (22) is then rewritten as ##EQU20##
As a result, the signals in the DSB and SSB regions 272 and 274 in the Q signal block are demodulated according to equations (17), (20), and (23). Since the baseband signal in the AM region 270 becomes 0 and is not present, a complete video signal cannot be obtained.
For this reason, the baseband video signal is extracted from the I signal block and is obtained from the output terminal 230 through the video signal filter 228.
If a television signal is received by the superheterodyne system, since the IF amplifier or the IF filter has characteristics of a Nyquist slope, the total baseband frequency characteristics are compensated to be flat. However, in this synchronous reception system, since such compensation is not performed, the video signal filter 228 substitutes for this compensation.
In the conventional television synchronous receiver described above with reference to FIG. 2, since the picture carrier frequency of the received channel is equal to the oscillation frequency of the voltage controlled oscillator (local oscillator), no image signals in the superheterodyne system are present. Therefore, the RF amplifier requires no tuned circuit, and the problem which is posed when a varactor diode is used as a tuning element is not present.
In addition, good frequency selectivity can be obtained by the low-pass filters. Furthermore, since a Costas loop as a synchronous carrier reproducing system is applied to the receiver, an input signal having a very low level can be directly demodulated.
In the arrangement shown in FIG. 2, however, if a television signal is present in the lower adjacent channel due to a special condition and place, the signal is folded on a received channel baseband signal, and interference shown in FIG. 4 is caused.
More specifically, in FIG. 4, reference symbol Vr denotes a received channel picture carrier; Ar, a received channel sound carrier; V.sub.-1, a lower-adjacent-channel picture carrier; and A.sub.-1, a lower-adjacent-channel sound carrier. When the lower-adjacent-channel sound carrier is converted into a baseband signal, it is folded and superposed on the 1.5 MHz signal in the received channel. As a result, fine beat stripes appear on the screen, and the picture quality is greatly degraded.
According to experiments conducted by the present inventor, it is found that if the level of a sound carrier in a lower adjacent channel carrier is lower than that of a picture carrier in a received channel in an excellent reception state by about 40 dB, no problems are posed in terms of picture quality.
In the conventional television synchronous receiver, interference from a lower adjacent channel cannot be rejected as long as a lower-adjacent-channel signal is not suppressed to be decreased in level by 40 dB or more in the RF amplifier. In practice, however, it is very difficult to selectively suppress a lower-adjacent-channel signal to decrease its level by 40 dB or more by using the tuned circuit in the RF amplifier.
That is, increasing the Q value and greatly narrowing a received channel band by arranging a tuned circuit in the RF amplifier will greatly degrade the noise figure and adversely affect the amplitude and phase characteristics.